1. Field of the Invention
The present invention relates to a laser apparatus and, more particularly, to a metal vapor laser apparatus using, e.g., copper as a laser medium.
2. Description of the Related Art
In recent years, a metal vapor laser has attracted attention as a light source used in uranium enrichment. In the uranium enrichment process, .sub.92.sup.235 U, serve which is used as a fuel for electric power generation by means of nuclear fission, is separated from natural uranium and is enriched. Since an abundance of .sub.92.sup.235 U is 0.7% in natural uranium, must be enriched to be about 3% in order to be use as nuclear fuel. For this purpose, in an uranium enrichment atom method using a laser beam, only .sub.92.sup.235 U is excited and ionized by a dye laser or the like, separated by an electrode applied with a voltage. In this case, the dye laser is a special laser in which in order to oscillate a dye laser beam, another type of laser beam is used. That is, in order to excite a laser medium of the dye laser, another type of laser beam is used. An example of the laser used for oscillating the dye laser beam is a metal vapor laser. An example of the metal vapor laser for dye laser excitation is a copper vapor laser.
FIG. 1 shows a conventional metal vapor laser apparatus. The apparatus shown in FIG. 1 has discharge tube 102, the interior of which is kept airtight, heat insulating member 106 which is inserted in tube 102, and cylindrical airtight vessel 104 which is arranged outside member 106, concentric therewith. Cylindrical core tube 108 is located inside member 106. A metal vapor source, e.g., a plurality of grains of copper materials 110 are located inside tube 108. Substantially annular electrodes 112 and 114 are located at both ends of tube 108 and member 106. Each of electrodes 112 and 114 has an L-shaped section along a plane including an optical axis. Electrodes 112 and 114 are connected to electric wires 116 extending from power source unit 118. In order to reliably insulate electrodes 112 and 114, from each other, annular high-voltage insulating member 120 is formed to be in contact with electrode 112 and vessel 104. Two transmission windows 122 for transmitting a laser beam are arranged at both ends of discharge tube 102 sealed by sealing members 123, respectively. In this case, each window 122 is arranged to form a Brewster angle with respect to the optical axis. High reflecting mirror 124 is arranged next to one of windows 122, to reflect a laser beam transmitted through this window, while output mirror 126 is arranged next to the other window 122. Cooling pipe 128 is wound around the outer surface of tube 102. Gas supply unit 130 is located at one end of tube 102, and vacuum pump 132 is located at its other end.
The conventional metal vapor laser apparatus having the above arrangement oscillates a laser beam as follows:
First, pump 132 is activated to evacuate tube 102 so as to obtain a negative pressure therein. A buffer gas is then supplied from gas supply unit 130 to tube 102, after which an electrical discharge is generated between electrodes 112 and 114 by, for example, a pulse voltage applied from power source unit 118. Copper materials 110 as a metal vapor source are heated by this discharge. Heated copper materials 110 is partially evaporated to be metal atoms and diffused in tube 102. In the above state, the buffer gas is ionized or excited by discharge. When the buffer gas collides against the metal atoms, energy is transferred to the metal atoms to excite them. The excited metal atoms transit to generate a laser beam. This laser beam is resonated and amplified between mirrors 124 and 126. As a result, a laser beam is emitted from mirror 126.
In the metal vapor laser apparatus which emits a laser beam as described above, core tube 108 is heated by electrical discharge between electrodes 112 and 114. The temperature of tube 108 is heated to a thousand and several hundreds degrees. Sealing members 123 are used, however, in order to airtightly mount windows 122 on tube 102. In this case, a material of sealing members 123 must be kept at a temperature of 200.degree. C. or less. For this reason, a very high temperature portion and a relatively low temperature portion are simultaneously present in the metal vapor laser apparatus. Therefore, the metal evaporated at the high temperature portion is solidified at the low temperature portion. In particular, since two windows 122 against which a metal vapor evaporated at tube 108 as the high temperature portion tends to collide are kept at a low temperature, the metal is solidified and adhered thereon. As a result, the transmission windows are contaminated by the metal vapor and their light transmittivity is reduced over time. If the transmittivity of the windows is reduced, an oscillation efficiency of a laser beam is reduced. This oscillation efficiency reduction poses a problem of reduction in service life of the metal vapor laser apparatus.
In order to solve the above problem, the transmission windows may be sufficiently separated from the core tube. In this method, however, the overall length of the discharge tube is increased to undesirably enlarge the apparatus. In addition, problems such as degradation in a short pulse laser beam quality or ASE (Amplified Spontaneous Emission) are posed.